(a) Field of the Invention
The present invention relates to a component for use in a gas turbine engine, particularly turbofan engines, which component is provided with means for reducing noise generated by said engine.
(b) Prior Art
The noise generated by contemporary turbofan engines has a significant economic impact. This is because aircraft operations are restricted to specific times during the day which prevents, for instance, 24-hour use of freight carriers. A sizeable portion of this noise is tonal in nature and can be partially attributed to the unsteady aerodynamic interaction between the stator and wakes shed from the upstream rotor. As one increases bypass ratio, fan noise is likely to be the dominant source.
Past methods for the reduction of fan tone noise have included choosing blade/vane counts such that the fundamental mode is cut off, increasing rotor-stator spacing, and the use of acoustic linings in the inlet and exit fan ducts. Some of these techniques however may not always be applicable. Duct liners suffer from the drawback that the thickness required to provide significant noise attenuation may be quite large, resulting in large weight increase at high bypass ratios.
In the quest for techniques aimed towards tone noise reduction, it is crucially important that their effect on the mean flow be minimal. In particular, the amount of flow turning in the exit guide vane should not be affected, nor the flow losses enhanced to any significant extent, for this would result in degraded performance. Thus, the system shown in U.S. Pat. No. 6,139,259 to Ho et al., wherein the pressure and suction surfaces of the exit guide vane (or strut) communicate through small channels over the entire leading edge surface of the airfoil, is not feasible since it would alter both the mean and unsteady loading.
It is desirable to develop a system capable of producing large (predictable) changes in the unsteady loading, while having little effect on the steady loading.